The present invention relates to precipitated silicas used as reinforcing fillers in vulcanizable rubber mixtures. Another aspect of the invention resides in the process for preparing certain precipitated silicas. A still further aspect of the invention resides in rubber formulations containing the precipitated silicas.
Precipitated silicas, in general, are known as is their use in tire formulations as shown by the following references.
S. Wolff, E. H. Tan: Performance of Silicas with Different Surface Areas in NR. Paper presented on the occasion of the ACS Meeting, New York City, N.Y., April 1986; and,
S. Wolff, R. Panenka, E. H. Tan: Reduction of Heat Generation Truck Tyre Tread and Subtread Compounds. Paper presented on the occasion of the International Conference on Rubber, Jamshedpur (India), Nov. 1986.
An essential parameter for characterizing a precipitated silica is the surface area, which is determined either by the adsorption of nitrogen (ISO 5794/1, Annexe D) or, of greater relevance to rubber engineering, by the adsorption of CTAB in accordance with ASTM D 3765-92. With reference to the rubber engineering data obtained, precipitated silicas are classified in six surface area classes in accordance with ISO 5794/1.
When applied to tires, however, almost exclusively precipitated silicas with CTAB surface areas between 100 and 200 m.sup.2 /g are used. See U.S. Pat. No. 5,227,425 and S. Wolff: The Influence of Fillers on Rolling Resistance, presented at the 129th meeting of the Rubber Division of the American Chemical Society, New York, N.Y., Apr. 8-11, 1986.
The reason for this is that precipitated silicas for use in tires must have high strength values and, especially in the region of the tread surface, must have good abrasion resistance. This can only be ensured by using precipitated silicas with surface areas in the range mentioned above. At present, precipitated silicas with CTAB surface areas &gt;200 m.sup.2 /g, on the other hand, are hardly ever used. However, it is precisely these precipitated silicas which should be characterized by a particularly high abrasion resistance and thus are of particular interest for practical application in tires. The essential reason why they are not used is due to the fact that precipitated silicas with increasing surface areas are extremely difficult to distribute (disperse) in a rubber mixture. This low capacity for being dispersed means that these products do not provide the general properties expected of them and thus offer no advantages, and sometimes even offer disadvantages, as compared with the precipitated silicas currently used in tires.
The reason for this poor dispersion of high surface area precipitated silicas is based on another important property of precipitated silicas, and that property is their structure (measured by means of DBP absorption in accordance with ASTM D 2414). Particularly important, however, is the distribution of macropore sizes (measured using Hg porosimetry in accordance with DIN 66 133) and how it changes with surface area; see the paper by U. Gorl, R. Rausch, H. Esch and R. Kuhlmann entitled: Kieselsaurestruktur und ihr Einflu.beta. auf das gummitechnische Wertebild, presented on the occasion of the German Rubber Conference in Stuttgart, June 1994.
In other words, the spread of macropore sizes generally decreases with increasing surface area, if it is not possible to produce additional large pores by means of appropriate precipitation techniques during the production of precipitated silica with the same surface area.
In particular, precipitated silicas with CTAB surface areas &gt;220 m.sup.2 /g require special precipitation techniques in order for them to have sufficiently large macropores, and thus good dispersion behavior, despite the large surface area.
It is known that precipitated silicas with the following physico-chemical parameters can be used in rubber mixtures (DE-A 44 27 137):
______________________________________ BET surface area 35 to 350 m.sup.2 /g Ratio of BET/CTAB surface areas 0.8 to 1.1 Pore volume PV 1.6 to 3.4 ml/g Silanol group density 6 to 20 ml (V.sub.2 = NaOH consumption) Average size of aggregate 250 to 1500 nm CTAB surface area 30 to 350 m.sup.2 /g DBP index 150 to 300 ml/100 g V.sub.2 /V.sub.1 by Hg porosimetry 0.19 to 0.46 preferably 0.20 to 0.23 DBP/CTAB 1.2 to 2.4 ______________________________________
An object of the invention therefore is to obtain precipitated silicas with CTAB surface areas of &gt;200 m.sup.2 /g which are characterized by a particularly high distribution of macropore sizes and thus by particularly good dispersion in a rubber mixture.
Another object of the invention is to obtain improved rubber formulations and products.